1

Dynamical Contribution to the Extratropical Precipitation Extremes

JIAN LU, L. RUBY LEUNG, QING YANG G. CHEN, W. COLLINS, FUYU LI, J. HOU AND X. FENG

Atmospheric Science and Global Change Division

Pacific Northwest National Laboratory, US Department of Energy

2

Background and Motivation

Circulation shifts/expands under global warming

Dynamical impacts on extratropical hydrological cycle

Possible convergence on the extratropical climate and extremes

DJF JJA

2020-2059

2060-2099

Increase of of 50-yr return level under scenario RCP8.5

Toreti et al. (2013)

5

Model and data

Model: NCAR CAM3, Spectral dycore, Aquaplanet configuration.

Simulations: T42, T85, T170, T340; 5 min time step, 26 levelsfor each resolution: control, 3K, sstgra

(only daily data at selected levels are available)

Li et al. (2011)

6

log10(PDF3K/PDFcntr), T42 log10(PDF3K/PDFcntr), T85

log10(PDF3K/PDFcntr), T170 log10(PDF3K/PDFcntr), T340

Convergence of Precip PDF

1/1000 events

7

Interpretation of PDF change

7

Shift in y direction Shift in p direction

= +

8

Minimization of frac variance

(in units of jet shift)

CC rate

(α)

9

Decomposition of precip pdf into dyn and thermodyn shifts

pdf diff due to the plwrd shift (45%)

C+D

Pdf diff dueto thermodyn(80%)

pdf diff due to 3K, normalized by clim pdf

[A] [C]

[D][B]

(PDF3K-PDFcntr)/PDFcntr (PDFshift-PDFcntr)/PDFcntr

(PDFth-PDFcntr)/PDFcntr

(90%)

1010

Alternative: scaling of the percentiles

dynamical “precip efficiency” column moisture

Convergence of effective diffusivity

(Marshall et al. 2006)

At large Pe (>200), Keff/k k-1

Keff const.

Sign of dynamical convergence

Keff/k

k−1

12

Conclusions

Sign of convergence emerges for certain range of extremes, at least within the modeling framework.

Circulation change exerts profound influence on the change of precip extremes by shifting the pdf poleward.

The poleward shift of the precip pdf can be accounted for by the poleward shift of the zonal mean zonal wind and the associated transients.

The sub-CC contribution from the thermodynamics hints at a reduction of “precipitation efficiency”.

Sign of dynamical convergence at resolutions > T170: effective diffusivity becomes independent of numerical diffusion coefficient.